ARM Template Parameters, Variables, and Outputs

Parameters, variables, and outputs make ARM templates flexible and reusable. Parameters accept input at deployment time, variables simplify repeated expressions, and outputs return values after deployment. In this lesson, you will learn how to use all three effectively.

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Parameters

Parameters allow you to provide values when deploying a template. This makes templates reusable across environments.

Defining Parameters

{
  "parameters": {
    "environment": {
      "type": "string",
      "allowedValues": ["dev", "staging", "prod"],
      "defaultValue": "dev",
      "metadata": {
        "description": "The deployment environment"
      }
    },
    "vmSize": {
      "type": "string",
      "defaultValue": "Standard_B2s",
      "metadata": {
        "description": "Size of the virtual machine"
      }
    },
    "adminPassword": {
      "type": "securestring",
      "metadata": {
        "description": "Administrator password for the VM"
      }
    },
    "instanceCount": {
      "type": "int",
      "defaultValue": 2,
      "minValue": 1,
      "maxValue": 10,
      "metadata": {
        "description": "Number of VM instances"
      }
    },
    "enableDiagnostics": {
      "type": "bool",
      "defaultValue": true
    },
    "tags": {
      "type": "object",
      "defaultValue": {
        "project": "webapp",
        "costCentre": "12345"
      }
    }
  }
}

Parameter Types

Type	Description
string	Text value
securestring	Sensitive text (not logged or displayed)
int	Integer number
bool	true or false
object	JSON object
secureObject	Sensitive JSON object
array	JSON array

Parameter Constraints

Constraint	Purpose
defaultValue	Value used when none is provided
allowedValues	Restricts input to a predefined list
minValue / maxValue	Numeric range limits
minLength / maxLength	String or array length limits

Referencing Parameters

Use the parameters() function to reference a parameter value:

"location": "[parameters('location')]",
"name": "[format('vm-{0}-{1}', parameters('environment'), parameters('vmSize'))]"

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Parameter Files

Instead of passing parameters on the command line, you can use a parameter file:

{
  "$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentParameters.json#",
  "contentVersion": "1.0.0.0",
  "parameters": {
    "environment": {
      "value": "prod"
    },
    "vmSize": {
      "value": "Standard_D4s_v3"
    },
    "instanceCount": {
      "value": 4
    },
    "enableDiagnostics": {
      "value": true
    },
    "tags": {
      "value": {
        "project": "webapp",
        "costCentre": "12345",
        "environment": "production"
      }
    }
  }
}

Deploy with a parameter file:

az deployment group create \
  --resource-group rg-webapp-prod \
  --template-file main.json \
  --parameters @parameters.prod.json

Key Vault References in Parameter Files

For secrets, reference Azure Key Vault directly:

{
  "adminPassword": {
    "reference": {
      "keyVault": {
        "id": "/subscriptions/<sub-id>/resourceGroups/<rg>/providers/Microsoft.KeyVault/vaults/<vault>"
      },
      "secretName": "vmAdminPassword"
    }
  }
}